A spark plug is known, which includes a center electrode, a ground electrode and a needle-like electrode tip joined to an inner surface (side surface) of an end of the ground electrode facing the center electrode so as to define a spark gap between the electrode tip and the center electrode. This type of spark plug allows the ground electrode to be located away from the spark gap and reduces the tendency that a flame core generated in the spark gap comes into contact with the ground electrode in the initial stage of flame growth as compared to conventional spark plugs. It is thus possible to decrease the quenching effect of the ground electrode and improve the ignition performance of the spark plug. This spark plug however decreases in spark wear resistance as the electrode tip gets heated to a high temperature. Against such a backdrop, Patent Document 1 proposes providing a core material of high thermal conductivity in the ground electrode of the spark plug so as to rapidly radiate heat applied from the electrode tip.
In the case of the above spark plug in which the needle-like electrode tip is joined to the ground electrode, the ground electrode needs to be longer by a length of the electrode tip than conventional ground electrodes. Although there has recently been a demand to reduce the size and diameter of the spark plug for engine downsizing and high engine performance, the diameter reduction of the spark plug leads to a smaller radial distance between the ground electrode and the spark gap than conventional ones. In order to prevent the ground electrode from interfering with the growth of the flame core in the spark gap, the radial distance between the ground electrode and the spark gap needs to be secured to some extent at a position corresponding to the spark gap. In other words, it is desired that the ground electrode not only secures an axially extending portion but also has a bent portion located as front as possible by increasing the bending degree of the bent portion (i.e. by decreasing the curvature radius of the inner surface of the ground electrode) in order to allow the end of the ground electrode to face the center electrode while preventing the ground electrode from interfering with the growth of the flame core.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2005-135783
However, it is likely that the internal stress developed in the bent portion of the ground electrode will increase as the minimum curvature radius of the inner surface of the bent portion becomes small. Further, the weight of the ground electrode increases with the length of the ground electrode so that the degree of internal stress developed in the bent portion by vibrations during engine driving becomes relatively high under the increased weight of the ground electrode as well as under the weight of the electrode tip joined to the end of the ground electrode. On the other hand, the heat radiation ability of the ground electrode decreases as the heat radiation passage of the ground electrode (i.e. the passage of heat radiation from the other end to the one end of the ground electrode and then to the metal shell) increases with the length of the ground electrode. There thus arises a possibility that the internal stress exceeds a fatigue limit to cause a breakage, particularly in the bent portion, in a state where the ground electrode decreases in metal fatigue strength under thermal load. This can result in deterioration of the breakage resistance of the ground electrode.